just as a heads up, i have g4 466, and my brother a 533, and both have 10.4 and ilife 05 installed completely, and working, without any problems, other then a little slow in imovie when encoding and the like. you can install and use them without a cpu upgrade. they came fresh installed on my mac when i bought it second hand, and thought that i'd have to upgrade the cpu soon to use the iapps, but its been close to a year now and i still haven't because it all works.
as far as the new cpus, the firmware does have to be installed from 9.2.2, which can be had from apple's website for free, as long as you have 9.1 installed. and it can be installed however long you want before the cpu, the patches are on their website, or were at one time anyway.
Digital Audio G4/1.467ghz, 1.5gig ram, 16x Superdrive, 256mb DDR3 AGP 6800GS, zip, 2x500gig raid0 for 1tb on sonnet tempo trio, 10.5.4